The DfT and TfL initially said this should be funded through a Private Finance Initiative deal – a contract with private sector companies to finance the trains. However, it was later agreed that public procurement was a better solution, and TfL will buy the trains at an estimated cost of £1 billion, funding it mainly through borrowing. DfT has contributed £100 million.

The NAO report said: “Crossrail will not be fully operational for another six years. Risks remain in a number of areas. For example awarding the contract to manufacture the trains by April 2014. Failure to do so could result in delays to services starting and a reduction in benefits. This process has suffered delays as a result of the decision to change the method of funding, but the sponsors are focused on achieving this date.”

It added: “The late decision to change funding of the rolling stock introduced a new delivery risk, but this is now being managed.”

A spokesman at TfL also told SM that everything is on track to meet the April deadline.